Growth and Electronic Structure of Organic Molecular Layers Studied by Density Functional Theory

Transcription

1 Growth and Electronic Structure of Organic Molecular Layers Studied by Density Functional Theory Slide 1

2 Motivation OLED para-sexiphenyl (6P) (C36H26) OFET Pentacene (5A) (C22H14) Slide 2

3 Outline 1. Density Functional Theory in a Nutshell 2. Van der Waals Forces: Surface / Adsorption Energies 3. Step-Edge Barrier in Organic Thin Film Growth 4. Orbital Densities from Angle-Resolved Photoemission 5. Dissecting Orbitals: Tomography in Reciprocal Space Slide 3

4 Outline 1. Density Functional Theory in a Nutshell 2. Van der Waals Forces: Surface / Adsorption Energies 3. Step-Edge Barrier in Organic Thin Film Growth 4. Orbital Densities from Angle-Resolved Photoemission 5. Dissecting Orbitals: Tomography in Reciprocal Space Slide 3

5 Density Functional Theory in a Nutshell Self-consistency Approximations: e.g.: LDA, GGA,... Slide 4

6 Cohesive Energy of Molecular Crystals Slide 5

7 Van der Waals Density Functional Nonlocal Correlation Energy leading to van-der-waals interaction Exchange-Correlation Energy Dion et al, Phys. Rev. Lett. 92, (2004). Slide 6

8 Outline 1. Density Functional Theory in a Nutshell 2. Van der Waals Forces: Surface/Adsorption Energies 3. Step-Edge Barrier in Organic Thin Film Growth 4. Orbital Densities from Angle-Resolved Photoemission 5. Dissecting Orbitals: Tomography in Reciprocal Space Slide 3

9 Cohesive Energy of Molecular Crystals Nabok, Puschnig, Ambrosch-Draxl, Phys. Rev. B 77, (2008). Slide 7

10 Surface Energies of Molecular Crystals 4A (100) 4A (0 10) 4A (0 01) 4A (110) γ [m J /m 2 ] Nabok et al. Phys. Rev. B 77, (2008); Ambrosch-Draxl et al., New J. Phys. 11, (2009). Slide 8

11 Thiophene / Cu(110) d Thiophene@Cu(110): Sony et al., Phys. Rev. Lett. 99, (2007). PTCDA@Cu,Ag,Au(111): Romaner et al., New. J. Phys. 11, (2009). Slide 9

12 Van-der-Waals-Bibliography Ab-initio vdw-density Functional Theory Dion et al, Phys. Rev. Lett. 92, (2004). Thonhauser et al., Phys. Rev. B, 76, (2007) Vydrov et al., Phys. Rev. Lett., 103, (2009) Lee et al. Phys. Rev. B 82, (2010) Efficient Implementations Roman-Perez et al. Phys. Rev. Lett. 103, (2009). Vydrov et al., J. Chem. Phys. 132, (2010). Lazic et al. Comp. Phys. Commun. 181, (2010). Nabok et al. Comp. Phys. Commun. 182, (2011). Applications huge number Brief Review see e.g.: Langreth et al., J. Phys.: CM 21, (2009) Slide 9

13 Van-der-Waals-Bibliography Ab-initio vdw-df Semi-Empirical Correction Dion et al, Phys. Rev. Lett. 92, (2004). Grimme, J. Comput. Chem., 25, 1463 (2004). Grimme, J. Comput. Chem., 27, 1787 (2006). Tkatchenko et al. PRL 102, (2009). Ruiz et al. PRL 108, (2012): PTCDA / Coinage-Metal(111)-Surfaces Slide 9

14 Outline 1. Density Functional Theory in a Nutshell 2. Van der Waals Forces: Surface / Adsorption Energies 3. Step-Edge Barrier in Organic Thin Film Growth 4. Orbital Densities from Angle-Resolved Photoemission 5. Dissecting Orbitals: Tomography in Reciprocal Space Slide 3

15 Island Growth on Amorphous Mica AFM-image 10 x 10 µm² T = 300 K θ = 0.32 ML F = 0.02 ML/min Amorphous Mica (ion bombarded) Observation of islands consisting of standing p-6p What is the critical cluster size? Transistion from lying-tostanding p-6p? Potocar et al., PRB 83, (2011). talk by A. Winkler, Tue 10:20-10:40 Slide 10

16 p-6p / p-6p(001) p-6p(001) as model substrate with weak Interactions top view side view Adsorption geometry: Slide 11

17 p-6p / p-6p(001) p-6p(001) as model substrate with weak Interactions top view side view Adsorption geometry: Energy landscape Diffusion path: b Slide 11

18 Lying vs. Standing p-6p DLA ALA critical cluster size i* Ebinding = Encluster / n Elying molecule Slide 12

19 2.6 nm Terraced Mounds AFM image: Sexiphenyl grown on a disordered mica surface Slide 13

20 Ehrlich-Schwoebel Barrier (ESB) Diffusion on a terrace Interlayer jump rate Slide 14

21 Sexiphenyl on Mica Ehrlich-Schwoebel Barrier = 0.67 ev = residence time (deposition time)2 AFM image: Film thickness = 30 nm 2nd layer nucleation rate Slide 15

22 Step-Edge Barrier Slide 16

23 Step-Edge Barrier in te rm o lecu la r in te ra ctio n s 1 2 e n e rg y co st fo r b e n d in g G. Hlawacek et al., Science 321, 108 (2008). Slide 17

24 Step-Edge Barrier Slide 17

25 Outline 1. Density Functional Theory in a Nutshell 2. Van der Waals Forces: Surface / Adsorption Energies 3. Step-Edge Barrier in Organic Thin Film Growth 4. Orbitals from Angle-Resolved Photoemission 5. Dissecting Orbitals: Tomography in Reciprocal Space Slide 3

26 Photoemission Spectroscopy Slide 18

27 Photoemission Intensity One Step Model Slide 19

28 Photoemission Intensity One Step Model molecular orbital Slide 19

29 Photoemission Intensity One Step Model molecular orbital plane wave ei k r Approximation: final state = plane wave Fourier Transform of Initial State Orbital [Feibelman and Eastman, Phys. Rev. B 10, 4932 (1974).] Slide 19

30 Comparison with DFT Molecular Orbital in Real Space z y x ky (1 /Å) k z(1/å) Calculation of the Fourier Transform kx (1/Å ) Slide 20

31 Comparison with DFT Hemispherical Cut Through 3D Fourier Transform Molecular Orbital in Real Space z y x k y(1/å) ky (1 /Å) k z(1/å) Calculation of the Fourier Transform kx (1/Å) kx (1/Å ) Slide 20

32 Comparison with DFT Hemispherical Cut Through 3D Fourier Transform Molecular Orbital in Real Space 2π/a k y(1/å) ky (1 /Å) k z(1/å) a~4.2 Å Calculation of the Fourier Transform kx (1/Å) kx (1/Å ) Slide 20

33 Toroidal Electron Energy Analyzer The Toroidal Electron Spectrometer for AngleResolved Photoelectron Spectroscopy with Synchrotron Radiation at BESSY II Slide 21

34 Sexiphenyl Monolayer on Cu(110) Slide 22

35 ky (1/Å) EF ky (1/Å) Constant Binding Energy Scans 2D-Momentum Maps CBE EHOMO=-1.9 ev CBE ELUMO=-0.4 ev kx (1/Å) Slide 23

36 2D-Momentum Maps ARPES Theory HOMO Filled LUMO Puschnig et al., Science 326, 702 (2009). Slide 23

37 Reconstruction of Orbitals ARPES IFT

38 Reconstruction of Orbitals HOMO Filled LUMO Puschnig et al., Science 326, 702 (2009). Slide 24

39 Outline 1. Density Functional Theory in a Nutshell 2. Van der Waals Forces: Surface / Adsorption Energies 3. Step-Edge Barrier in Organic Thin Film Growth 4. Orbital Densities from Angle-Resolved Photoemission 5. Dissecting Orbitals: Tomography in Fourier Space Slide 3

40 ARPES of PTCDA / Ag(110) STM Glöckler et al, Surf. Sci. 405, 1-20 (1998). Slide 25

41 Identifying Orbitals Self-interaction corrected functional Slide 26

42 HOMO and Filled LUMO M1= FLUMO M2= HOMO Puschnig et al. PRB 84, (2011), Ziroff et al., PRL 104, (2010). Slide 27

43 What is the nature of M3? M1= FLUMO M2= HOMO? Slide 28

44 ARPES Data-Cube Eb ky kx Slide 29

45 π- Bands of PTCDA Slide 30

46 What is the Origin of M3? E C F Slide 31

47 What is the Origin of M3? E C F Puschnig et al. PRB 84, (2011), see also: Dauth et al., PRL 107, (2011). Slide 32

48 Projected DOS from ARPES! C Eb (ev) Fit parameters = PDOS D calculated orbitals E measured photemission data cube F Slide 33

49 Ag-row direction [1-10] Benchmark for Theory C D E F Slide 34

50 Identification of Orbitals Mixed Monolayers ARPES Tomography PTCDA STM CuPc Slide 35

51 Conclusion Van der Waals Interactions within DFT Organic / organic works fine; organic / metal interactions more problematic Nabok et al., PRB 77, (2008). Sony et al., PRL. 99, (2007). Romaner et al., NJP 11, (2009). Island Growth and Step-Edge Barriers Critical cluster = 2-3, Transition lying standing p-6p about 15 molecules Potocar et al., PRB 83, (2011). Some success in understanding certain kinetic barrieres, but still a lot of work to do... G. Hlawacek et al., Science 321, 108 (2008); see also: Goose et al., PRB 81, (2010). Orbital Densities and Hybridization with Metallic States Puschnig et al., Science 326, 702 (2009); Ziroff et al., Phys. Rev. Lett. 104, (2010). Berkebile et al., Phys. Chem. Chem. Phys. 13, 3604 (2011). Orbital Tomography [Puschnig et al. PRB 84, (2011)] Make use of characteristic momentum space patterns Unambiguous identification of molecular features Density of states projected onto molecular orbitals Deconvolution beyond limits of energy resolution Slide 37

52 Collaborations and Funding Lehrstuhl für Atomistic Modelling and Design of Materials MU Leoben Dmitrii Nabok, Priya Sony, Lorenz Romaner, Claudia Ambrosch-Draxl Institut für Physik, Montanuniversität Leoben, Austria Gregor Hlawacek, Stefan Lorbek, Quan Shen, Christian Teichert Theoretical Physics University Graz, Austria Daniel Lüftner, Matus Milko, Peter Puschnig Experimental Surface Science Group University Graz, Austria Thomas Ules, Eva-Maria Reinisch, Stephen Berkebile, Alexander Fleming Georg Koller, Mike Ramsey Institut für Festkörperphysik, TU Graz, Austria Thomas Potocar, Paul Frank, Adolf Winkler Peter Grünberg Institut (PGI-3), JARA, Forschungszentrum Jülich, Sergey Soubatch Stefan Tautz The work is part of the National Research Network Interface controlled and functionalized organic films and the single project P N16 Understanding photoemission of organic thin films Slide 36

53 Layer-Dependent ESB ESB 0.26 v s

54 Layer-Dependent ESB ab in itio DFT E xpe rim e nt S im u la tio n E m p irica l po te ntials G. Hlawacek et al., Science 321, 108 (2008).

55 ARPES Map of σ-orbital